Novel filter design tackles motor damage from inverters

04 November, 2007

REO Inductive Components claims to have come up with a way of overcoming the problem of high-frequency, common-mode disturbances in inverter-driven motor installations. Such disturbances can cause capacitance coupling through the motor’s stator, rotor, bearings and frame, and can damage the motor.

Frequency inverters with switching frequencies in the range 2–20kHz cause considerable common-mode (asymmetrical) noise. The rapid switching of the DC voltages causes voltage peaks in the motor’s cables: the longer the cable, the bigger the peaks.

The fast-rising voltages (dV/dt) contain many harmonic frequencies, and can cause ripple on the voltage sinewave between line and earth. In the most severe cases – especially when there are long cable runs between the inverter and motor – this can damage the bearings or insulation, leading to premature motor failures. Other problems associated with such installations include high noise levels, power losses and parasitic earth currents.

The traditional answer is to fit a low-pass filter to the inverter’s output terminals (as shown above), thus reducing the differential mode interference and producing a smooth sinusoidal waveform with a ripple content of less than 5%.

But this approach is not always enough, especially in applications where there are high-frequency, common-mode disturbances, and where the motor would be costly to replace or plays a critical role.

REO’s new technique (above) is aimed at such applications and involves the combined use of differential-mode and common-mode output filters. Called the Sinus Plus++ filter, it provides a means of allowing the common-mode interference to flow back to the DC link instead of through capacitive coupling to earth. As well as protecting critical motors, this technique can also be used where leakage currents in parallel connected circuits are causing interference.

REO claims that this approach also delivers other benefits:º it allows unlimited cable lengths between the inverter and motor;º it allows the use of switching frequencies above 8kHz;º it reduces audible noise;º additional filtering on the mains supply side is not needed;º it reduces cable and eddy current losses, allowing smaller drives to be used;º radiated interference from the motor cables is minimised, and unshielded cables can be used; andº hazardous voltages and leakage currents are avoided.

One typical application for the new filter is to protect the bearings of underground pump motors that need long cable runs and would be costly to repair or replace. Another example is in multi-storey car parks, where several ventilation fans are connected in parallel to a one frequency inverter, requiring long cable runs and risking capacitive coupling between the cables.